Measurements of lighting
It is the density of photons which fall within a given surface area. It is measured in lux or lumens/m2 and can be measured with the help of a lux meter. For a given light source, the closer to a light source the illuminated area is, the higher the Illuminance value.
Illuminance is used in lighting applications to measure the amount of light reaching an object such as a wall. The illuminance will decrease if the total area illuminated is increased without changing the intensity of the light source, although the luminance intensity of the light source stays the same.
It is the density of luminous intensity in a given direction and falls within a given solid angle. It is measured in cd/m2. It is often used to characterize the emission from a diffuse surface. It tells how much luminous power will be perceived by eye when viewing the surface from a particular angle. Luminance remains the same regardless of the distance from the light source. In contrast to illuminance i.e. light falling on a surface, luminance is a measure of light coming from a surface.
Luminance does not follow a square law, but the measurement area must be defined. For example, luminance of a wall remains same whether measured two meters away or four. Likewise, moving closer to or further away from a source document does not change its apparent brightness.
Particular luminance of a bright surface is generally referred to as its "brightness" because it is the quality of brightness that we perceive. Brightness, also known as luminance or photometric brightness is the luminance intensity of any surface in a given direction. It is measured per unit of projected area of the surface as viewed from a single direction.
Lux (lx) is the SI derived unit of illuminance or illumination. It is equal to one lumen per square metre.
- Sunlight on an average day ranges from 32 000 to 100 000 lux
- TV studios are lit at about 1 000 lux [i.e. 1000 lumens per square metre]
- A bright office has about 400 lux of illumination
- At sunset and sunrise, ambient outdoor light is also about 400 lux (if the sky is clear).
- Moonlight represents about 1 lux
- Starlight measures a mere 0.00005 lux
Lumens measure "luminous flux". This is a measure of the total number of packets or quanta of light produced by a light source for example a globe or fluorescent tube. This is the quantity of light emitted by the light source. The purpose of lux is intended to tell us how many lumens we need, given the area we are trying to illuminate.
The difference between the lux and the lumen is that the lux takes into account the area over which the luminous flux is spread. 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produce a dimmer illuminance of only 100 lux.
Wattage of a light source refers to the power consumed to drive the source while lumens refer to the brightness of that source as the human eye perceives it. The wattage of course would be the sum of the heat generated as well the energy of the light emitted.
In general, Mean lumens are the number of lumens produced at 40 to 50 percent of rated lamp life.It is basically the Lumen output of a light source after the source has been used. Mean lumen values for fluorescent and HID lamps are typically measured at 40% of their rated lives. Most high pressure sodium and mercury lamps are measured at 50% of their rated lives. All measurements are made on ANSI reference ballasts. Mean lumens are not typically measured for incandescent and tungsten halogen lamps.
It is the number of lumens produced by a light source for each watt of electrical power supplied to the light source. It is a measure of the efficacy (efficiency) of lamps which indicates the amount of light (lumens) emitted by the lamp for each unit of electrical power (watts) used.
It is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1⁄683 watt per steradian. An ordinary candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not masked.
It is defined as the amount of illumination the inside surface of a 1-foot radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere. Alternatively, it can be defined as the illuminance on a 1-square foot surface of which there is a uniformly distributed flux of one lumen. This can be viewed as the amount of light that actually falls on a given surface.The foot-candle is equal to one lumen per square foot. It is a non-SI unit of illuminance or light intensity widely used in photography, film, television, and the lighting industry. It has been replaced in the International System by the candela i.e. 1 lumen per square meter.
Nowadays, Candlepower is an obsolete unit. However, earlier it was used to express levels of light intensity in terms of the light emitted by a candle of specific size and constituents. In modern usage Candlepower equates directly to the unit known as the candela.
The candlepower of a lamp was measured by judging by eye the relative brightness of adjacent surfaces, one illuminated only by a standard lamp or candle and the other only by the lamp under test. The distance of one of the lamps was adjusted until the two were judged to give equal brightness. The candlepower of the lamp under test could then be calculated from the two distances and the inverse square law.
Luminous efficacy is a figure of merit for light sources. It is the ratio of luminous flux emitted from a light source to the electric power consumed by the source, and thus describes how well the source provides visible light from a given amount of electricity. In other words, it is the ratio of luminous flux (in lumens) to power (in watts). It is usually expressed in lumens per watt.
Luminous efficacy, when expressed in dimensionless form i.e. as a fraction of the maximum possible luminous efficacy, may be called overall luminous efficiency, wall-plug luminous efficiency, or simply the lighting efficiency. It is normally expressed as percentages.